Is there a chance you could upload those changes to production or send me a link to your dev environment so I can take a closer look? If you would rather not make the dev address public, you can send it to me to `
[mynickname].wp{at}Google_Mail{dot}com
`
Cheers!

Just in case someone else finds this thread and is experiencing the same error, the original validation error related to the <body> was caused by improperly placed HTML outside of the body tag. The culprit seems to be a plugin, but I could not verify it for sure.